Monday, October 11. 2010

Some of you have been following along at home as we strive to understand and restore the carbon arc headlights on this locomotive. See the submitted BLOG comments for some good information and discussion.

John Nelligan has researched and come up with some very good 'new to us' information. I will repost his findings below. Thank you John and Steve Kraus, for making all of us a little bit smarter. Reading John's findings in light of Steve Kraus's comments makes sense. The brightness of a carbon arc light device requires the positive electrode placement to be just so, to give maximum efficiency. Reversing polarity with the DPDT headlight switch makes the other electrode the positive side, and it is now outside the optimum placement, making the headlamp seem 'dimmer' in that connection scheme. The arc itself is unchanged.

Steve's experience is with lower voltage arc lights. In the locomotive, the circuit receives a nominal 600 volts, but with the dropping resistance, the effective potential across the headlight is MUCH lower, after the arc is struck. In fact, maybe VERY MUCH lower since ionized air in an established arc provides almost ZERO resistance.

"Last Thursday, I got lucky when I googled carbon-arc headlights and found a
1909 GE bulletin describing exactly what is installed in the CCW 300 for
headlight operation. The venerable GE engineers discovered the when the
terminals of the headlight are hooked up with one polarity, the light emitted is
bright (1800' down the track) and when the polarity is reversed, the light is
not nearly as bright and suitable for street running in urban locales. Thus the
use of a DPDT (double pole double throw) switch that has been marked 1200 (incorrectly as has been observed). The only workable
configuration for the headlight resistance is to tap it with the same number of
resistors on each side of the tap pair. The current along each of the 2 equal
paths will be 3 Amps when 600 Volts is applied to the taps. This
arrangement serves both positions of the DPDT switch. There is no possible tap
arrangement that would not fry the resistor bank, were 1200 Volts to be applied across the taps."

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